Conservatory air conditioning: what works and what's a waste of money

Updated July 2026

A conservatory is a greenhouse you've put furniture in. That's not an insult — it's the physics you're up against. Glass admits the sun's shortwave radiation freely, the contents absorb it and re-radiate it as heat that glass traps, and the result is a room that hits 35–40°C on a day when the garden is 26°C. Cooling one is genuinely the hardest residential job in Britain, which is why it attracts more snake oil than any other room. Here's the honest hierarchy.

What's largely a waste of money

• Evaporative "air coolers". They cool by adding moisture to the air — in a sealed glass box that's already humid, they make the room feel worse. In a conservatory they're close to useless, whatever the listing says.
• Undersized portables. A 9,000 BTU portable that copes with a bedroom will lose the fight against midday solar gain through 20 m² of glass. It runs flat out, leaks heat back through its hose, and takes the edge off at best. If a portable is your only option, size aggressively (12,000 BTU+), run it from mid-morning before the heat builds, and treat it as damage limitation.
• Fans alone. Above ~30°C — which a conservatory passes by 11am — moving hot air around achieves little.

What helps (and is worth doing regardless)

Stop the sun before it enters. Every pound spent blocking solar gain outperforms a pound spent removing heat afterwards. External shading (sail shades, awnings) beats internal blinds because it rejects heat before the glass; solar-control window film on the roof glazing cuts gain meaningfully for £100–£300 of materials; and roof blinds help even though they act after the glass. Ventilate strategically: open roof vents and door early morning and evening, close everything (with shading deployed) before the day heats up. These measures might bring a 38°C conservatory down to 30°C — a big improvement that's still not comfortable. Which is where actual cooling comes in.

What actually works: a properly sized split system

A wall-mounted split is the only technology that reliably makes a conservatory usable on a hot afternoon, and it comes with conservatory-specific caveats your installer should address on survey:

• Size for the glass, not the floor area. A 15 m² conservatory can carry the heat load of a 30 m² normal room. Run the sizing calculator with sun exposure on "sunny" as a floor, and expect the installer's survey to size at or above that — 3.5 kW is a common answer for a mid-sized conservatory where the same-sized lounge would take 2.5 kW.
• Mounting needs a solid wall. Glass and uPVC frames can't carry a wall unit. The usual solutions: the house wall the conservatory backs onto (often ideal — unit faces into the room), a low-level console unit on the dwarf wall, or a compact ceiling cassette in solid-roof conversions.
• The winter payoff is bigger here than anywhere. The same glass that overheats the room in July makes it fridge-cold from October to April. A reversible split turns a three-season liability into a year-round room for 10–50p an hour — for many owners that, not summer cooling, is what justifies the spend.

Cost-wise, expect the normal single-split range — £1,500–£3,500 fitted (see the cost guide) — with sizing and any access complexity deciding where in the range you land. Planning is rarely an issue for a reversible unit on a house since the May 2025 rule change (details here).

The sensible combined strategy

Shading and film first (cheap, permanent, reduces the AC size you need), then a split system sized for the remaining load, run with the shading deployed. That combination cools for materially less per hour than brute-forcing an unshaded glass box — and the system you buy can be a size smaller.

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